max31855: reject personality above 15 at load time

[grandixximo]

Follow-up to the discussion on #4302.

max31855 sizes its sensor pins by (personality & 0xf) but indexes them with the raw value capped at 15. personality=16 exports zero sensor pins while the function accesses 15 of them, dereferencing NULL pin pointers and killing rtapi_app as soon as the function runs in a thread.

Repro (uspace sim, RIP build):

loadrt threads name1=t period1=100000 fp1=0
loadrt max31855 personality=16
addf max31855.0.bitbang-spi t
start

Before: rtapi_app dies within one thread period of start. Silent in POSIX fallback: halrun still exits 0 and halcmd keeps answering from shm; only pgrep -x rtapi_app shows it.
After: loadrt fails with max31855: personality must be at most 15 (use personality=N to set the number of sensors).

Also adds the agreed comp.adoc note: components with personality-sized pins should validate the value in EXTRA_SETUP and return -EINVAL, with an example.

[BsAtHome]

These are very long sentences. Maybe split it up in smaller sentences.
It also ends with a colon?

[grandixximo]

Split in shorter sentences.
The sentence was preamble to the code, the colon was the glue, made it more specific.

[grandixximo]

See if it reads better now, or did I overdo it?

[BsAtHome]

What about:

[NOTE]
----
If a component uses personality, then it should generally check its value.
The value of personality is zero if the 'personality=N' argument is not provided to loadrt.
Any unconditional access to pins or params created by personality constraints, when zero, causes a crash.
Pins and params are not created when personality is zero and its memory remains NULL.
Accessing a not created pin or param accesses a NULL pointer, which inevitably leads to a crash.

You can use a test in EXTRA_SETUP() to test the acceptable values of personality for your component.
You should return -EINVAL if your conditions are not met.

Example testing personality:
[source,c]
...

----

[BsAtHome]

A real bikeshed moment :-)

[grandixximo]

Continuing the bikeshed painting: I kept your first two sentences and the remedy paragraph verbatim, and merged the three middle ones into a cause-then-effect pair, since they said "crash" twice and "a not created pin" read clunky to me. I feel it reads better like this, but your version is acceptable to me too if you prefer it.

[NOTE]
====
If a component uses personality, then it should generally check its value.
The value of personality is zero if the 'personality=N' argument is not provided to loadrt.
Pins and params whose personality constraint evaluates to zero are not created, and their memory remains NULL.
Unconditionally accessing such a pin or param dereferences a NULL pointer and crashes the realtime process.

You can use a test in EXTRA_SETUP() to test the acceptable values of personality for your component.
You should return -EINVAL if your conditions are not met.

Example testing personality:

[source,c]
----

[BsAtHome]

I like your color too.